This invention relates to a process for recovering high-purity benzyl benzoate from a reaction mixture obtained by the oxidation of toluene by molecular oxygen.
Benzyl benzoate is an important compound useful as raw material for perfumes and pharmaceuticals and applicable to other industries and a large number of processes for its manufacture have been reported. For example, Japan Kokai Tokkyo Koho Sho 56-39045 (1981) reports the recovery of benzyl benzoate by distillation of benzyl benzoate under reduced pressure from a reaction mixture produced by the oxidation of toluene by molecular oxygen. However, distillation alone has not yielded benzyl benzoate with such high purity as to be acceptable for use in perfumes. Likewise, Japan Kokai Tokkyo Koho Sho 53-95934 (1978) reports the recovery of benzyl benzoate by distillation from a reaction mixture produced by the oxidation of toluene by molecular oxygen, but high-purity benzyl benzoate is not obtained on account of the difficulty of separating fluorenone whose boiling point is close to that of BZB.
Japan Kokai Tokkyo Koho Sho 53-31639 (1978) describes a process for synthesizing benzyl benzoate by the transesterification of methyl benzoate with benzyl alcohol and an article in Industrial and Engineering Chemistry, Vol. 39, No. 10, pp.1300-1302 (1947) describes the synthesis of benzyl benzoate by the reaction of sodium benzoate with benzyl alcohol. Although these processes yield high-purity benzyl benzoate, they face a problem of high manufacturing cost because of the use of expensive catalysts and raw materials.
An object of this invention is to provide a process, free from the aforementioned shortcomings, for recovering benzyl benzoate in high purity at low cost.
This invention relates to a process for recovering benzyl benzoate which comprises distilling the residue remaining after removal of unreacted toluene and benzoic acid from a reaction mixture produced by the oxidation of toluene by molecular oxygen in the presence of a metal catalyst thereby separating a fraction of benzyl benzoate containing 80% by weight or more benzyl benzoate and simultaneously 10% by weight or less benzoic acid and 10% by weight or less fluorenone, purifying the fraction of benzyl benzoate by crystallization and recovering high-purity benzyl benzoate.
Benzyl benzoate (BZB) is known to be present as by-product in a reaction mixture produced by the oxidation of toluene by molecular oxygen in the presence of a catalyst. In general, the oxidation of toluene is carried out for the purpose of manufacturing benzoic acid and those catalysts which are commonly used for the reaction are based on heavy metals such as cobalt and manganese. As for the normal operating conditions inside the reactor, the temperature is 120-170xc2x0 C., the pressure is normal to 1 MPa and air is frequently used as molecular oxygen. A reaction mixture produced under any conditions, not limited to the aforementioned conditions or the purpose, may be used in this invention as long as it contains BZB and benzoic acid.
Either a continuous process or a batch process may be chosen for the oxidation reaction, though a continuous process is more economical. The main product to be produced by the oxidation of toluene is benzoic acid, but the following by-products besides unreacted toluene have been identified in the reaction mixture: water, formic acid, acetic acid, propionic acid, acetone, benzene, biphenyl, methylbiphenyl, dimethylbiphenyl, fluorene, fluorenone, toluic acid, phthalic acid, isophthalic acid, terephthalic acid, biphenylcarboxylic acid, methylbiphenylcarboxylic acid, biphenyldicarboxylic acid, and phenylbenzyl alcohol, compounds occurring as intermediates in the oxidation of toluene to benzoic acid such as benzaldehyde and benzyl alcohol, and esters formed by the esterification of benzyl alcohol or phenylbenzyl alcohol with a variety of carboxylic acids inside the reactor such as benzyl benzoate (BZB), benzyl acetate (BZA), benzyl toluate and benzyl biphenylcarb oxylate.
The reaction mixture is successively distilled to recover unreacted toluene and the main product benzoic acid. Simultaneously distilled in this step are components boiling lower than toluene such as water, formic acid, acetic acid, propionic acid, acetone and benzene and an intermediate fraction with a boiling point between those of toluene and benzoic acid such as benzaldehyde and benzyl alcohol. The unreacted raw material toluene and the intermediates benzaldehyde and benzyl alcohol are returned to the reactor for conversion to benzoic acid. Components boiling higher than benzoic acid, for example, biphenyl, methylbiphenyl, dimethylbiphenyl, fluorene, fluorenone, toluic acid, phthalic acid, isophthalic acid, terephthalic acid, biphenylcarboxylic acid, methylbiphenylcarboxylic acid, biphenyldicarboxylic acid, benzyl benzoate (BZB), benzyl acetate (BZA), and benzyl toluate are contained in the residue after the distillation of benzoic acid. Although the composition of the high-boiling components contained in the residue varies with the conditions for reaction and distillation, any residue containing at least a given amount, preferably 5% by weight or more, of BZB is acceptable for use in this invention. The residue may contain heavy metals such as cobalt or manganese originating from the oxidation catalyst, but it is advantageous to extract such heavy metals by hot water.
The BZB contained in the residue is distilled again and recovered as a BZB fraction. Either a continuous process or a batch process may be chosen for the distillation, though a continuous process is preferable economically. The BZB fraction obtained here is purified in the recrystallization step. Economical recovery of high-purity BZB, however, makes it necessary to maximize the recovery and the concentration of the BZB fraction in the distillation step.
The distillation in question is carried out below 270xc2x0 C., preferably below 250xc2x0 C., in order to prevent benzoate esters from thermally decomposing to benzoic acid as much as possible when the temperature at the bottom of the distillation column becomes too high and also to prevent benzoic acid from corroding the apparatus. As the boiling point of BZB is 323xc2x0 C., it is desirable to carry out the distillation under reduced pressure while keeping the pressure at the top of the distillation column preferably at 4.0 Kpa or less.
The number of plates for a distillation column used for the recovery of BZB from the residue will be 5 or more for satisfactory operation and the concentration of the BZB fraction can be controlled by reflux ratio. Providing the distillation column with an unnecessarily larger number of plates will raise the temperature at the bottom of the column on account of the pressure difference inside the column and occasionally cause such problems as formation of by-product benzoic acid and corrosion of the apparatus. The concentration of BZB in the BZB fraction to be obtained by this distillation is preferably 80% by weight or more, more preferably 90% by weight or more, in consideration of the recovery and purity in the following crystallization step. For this reason, the BZB fraction is obtained by carrying out the distillation preferably at 180-220xc2x0 C., more preferably at 190-210xc2x0 C., in case the pressure at the top of the distillation column is kept at 2.6 KPa.
The BZB fraction contains, in addition to BZB, such compounds as benzoic acid and fluorenone whose boiling point is close to that of BZB. In case the concentration of BZB in the BZB fraction is less than 80% by weight, the recovery of BZB in the following recrystallization step falls and, at the same time, benzoic acid and fluorenone crystallize together with BZB to lower the purity of the recovered BZB. The presence of less than 10% by weight of benzoic acid in the BZB fraction allows economical recovery of BZB while the presence of benzoic acid in excess of 10% by weight decreases the recovery of BZB in the crystallization step thereby making economical recovery of BZB difficult. In the presence of less than 10% by weight of fluorenone in the BZB fraction, BZB purified in the recrystallization step is colorless and transparent. On the other hand, when fluorenone is present in excess of 10% by weight in the BZB fraction, it crystallizes simultaneously with BZB and, as a result, BZB becomes colored yellow and unsuitable as raw material for perfumes and pharmaceuticals.
The BZB fraction is purified by crystallization to a purity acceptable as raw material for perfumes and pharmaceuticals. Known processes such as recrystallization, recrystallization and solvent washing, dispersive crystallization in solvent, multi-stage crystallization and continuous crystallization can be adopted for purification by crystallization. Advantageous among them is continuous crystallization and recrystallization, crystallization and solvent washing or dispersive crystallization in solvent. Recrystallization is carried out advantageously by dissolving BZB in a solvent that dissolves BZB moderately and is readily separable from BZB and then recrystallizing the BZB. In case the BZB fraction is obtained as liquid, the fraction is dissolved as it is in a solvent and allowed to recrystallize. Any solvent is suitable for this recrystallization if it shows a good solvent power against the BZB fraction above 10xc2x0 C. but such an insufficient solvent power against BZB below 10xc2x0 C. as to allow only BZB to crystallize and boils lower than BZB. Such solvents include acetone, methyl isobutyl ketone, methyl ethyl ketone, methanol, ethanol, isopropanol, butanol, tetrahydrofuran, dioxane, benzene, toluene, xylene, ethylbenzene, cyclohexane, methylcyclohexane, ethylcyclohexane, dimethyl ether, diethyl ether and petroleum ether. Two or more of such solvents may be mixed properly or an adequate amount of water may be added to these solvents. An aliphatic alcohol with 1 to 5 carbon atoms such as methanol, ethanol, propanol, butanol and pentanol or its mixture with water is a highly efficient solvent for the purification of BZB. In the case of a mixture of an alcohol and water, the alcohol to water ratio is preferably in the range from approximately 1:2 to 2:1.
Crystallization and solvent washing is effected by washing with a solvent the crystals formed by cooling the BZB fraction. A preferable procedure for washing is dispersing the crystals in a solvent followed by solid-liquid separation or to adding the BZB fraction to a solvent, allowing BZB to crystallize by cooling and submitting the resulting mixture to solid-liquid separation. In the course of crystallizing the BZB fraction by cooling, adding a solvent in a small amount or setting the crystallization temperature at a slightly higher level helps to form crystals of higher purity or facilitate filtration. Dispersive crystallization in solvent is preferably effected by adding the BZB fraction or the crystals thereof formed by cooling to a solvent to form a dispersion of the crystals. In either case, any solvent may be used satisfactorily as long as it dissolves the impurities present in the BZB fraction and it is not always necessary for the solvent in question to have an ability to dissolve BZB. As the impurities and BZB behave similarly in solvent solubility, however, a solvent capable of dissolving BZB is used. Preferable solvents here are the same as the aforementioned solvents for recrystallization. Whenever the purity falls short of the target level or there occurs a need to remove the adhered solvent after completion of the aforementioned purification by crystallization, an after-treatment is performed in the same as in the case of recrystallization to be described below.
Recrystallization is effected by dissolving the BZB fraction or the crystals obtained therefrom in a solvent and cooling the resulting solution to a temperature below the melting point of BZB, preferably to below 10xc2x0 C., to separate the crystals. Cooling to below 5xc2x0 C. is preferable for an efficient recovery of BZB. However, cooling to below xe2x88x9230xc2x0 C. is not desirable as this will occasionally cause components other than BZB to separate. Use of 5 parts by weight or less of the solvent per 1 part by weight of the BZB fraction is adequate. An excess of the solvent relative to the BZB fraction is undesirable because the recovery of BZB during recrystallization decreases. It is to be noted that the BZB fraction is liquid not only at the time when it distills out but also usually at normal temperature and the fraction as liquid is occasionally dissolved in a solvent. In this invention, dissolving the BZB fraction and crystallizing it in this manner is also referred to as recrystallization. Recrystallization is preferably effected with stirring and the separated crystals of BZB are collected by a procedure for solid-liquid separation such as centrifugal filtration and vacuum filtration.
Since the mother liquor containing the impurities adheres to the crystals, it is necessary to wash the crystals with the same solvent as used in the recrystallization step to obtain high-purity BZB. The crystals can be washed either by adding a washing solvent directly to the surface of the collected crystals or by throwing the crystals in a solvent and filtering them again. In either case, washing of 1 part by weight of the crystals requires 0.5 part by weight or more of the washing solvent. Washing with less than 0.5 part by weight of the solvent cannot remove the impurities in the mother liquor sufficiently and the purity of BZB deteriorates. On the other hand, washing with too much solvent decreases the recovery of BZB, which is not economical.
The solvent-washed crystals are melted by heating to above 18xc2x0 C., which is the melting point of BZB, and the adhered solvent is distilled off under reduced pressure. In this manner, it is possible to recover economically colorless transparent BZB with a purity of 99.5% or more, useful as raw material for perfumes and pharmaceuticals and applicable to other industrial uses.